Terephthalic acid is produced through liquid-phase oxidation of a p-phenylene compound such as p-xylene or the like, in which, in general, used is a catalyst such as cobalt, manganese or the like, or a catalyst further added with a promoter such as a bromine compound, acetaldehyde or the like, in the presence of an acetic acid solvent.
A slurry containing terephthalic acid produced through such liquid-phase oxidation is, in general, processed for crystallization by lowering the temperature thereof, and then further processed for solid-liquid separation under a pressure close to ordinary pressure to give a terephthalic acid cake.
On the other hand, the oxidation reaction mother liquid separated through the solid-liquid separation contains useful catalyst components such as a heavy metal ion, a bromide ion or the like derived from the catalyst, and in industrial operation, the production cost must be reduced by recycling these catalyst components.
A simplest recycling method includes directly returning the oxidation reaction mother liquid to the reaction system for reusing it therein, and is widely employed in an industrial-scale production process. However, the oxidation reaction mother liquid is contaminated with various organic impurities formed as by-products and inorganic impurities derived from plant corrosion; and it is known that, when the oxidation reaction mother liquid is directly reused in the reaction system, then the concentration of these impurities in the reaction system may gradually increase, and when the concentration exceeds a certain level, then it would have some negative influences on the liquid-phase oxidation reaction.
For example, in case of terephthalic acid, it is said that the proportion of the oxidation reaction mother liquid to be returned to the reaction system is generally from 70 to 98%, and the remaining oxidation reaction mother liquid of from 2 to 30%, which is not reused in the reaction system, is fed to a step of recovering the solvent, acetic acid.
As a method for recovering and reusing the catalyst components from the oxidation reaction mother liquid fed to the step of recovering the acetic acid, there has been proposed a method of using a pyridine ring-containing chelate resin (see PTL 1).
As described in PTL 1, since the pyridine ring-containing chelate resin is, in general, not used in an ordinary water solvent system but used in an acetic acid solvent system, the chelate resin needs to be replaced with an acetic acid solvent in advance, and, in addition, since a bromide ion exists in a high concentration in the oxidation reaction mother liquid, the chelate resin needs to hold a bromide ion as an anion. The condition where the resin holds a bromide ion is hereinafter referred to as Br− form.
However, it has become clear that, when a pyridine ring-containing chelate resin containing water as a solvent is brought into contact with an acetic acid solvent, then there occur phenomena unfavorable for pretreatment operation such as the swelling of the resin, heat generation of the resin and air bubbles generation.
The swelling of the pyridine ring-containing chelate resin is caused by change in the solvent-containing condition of the resin resulted from the replacement of the water solvent taken inside the resin with the acetic acid solvent. In fact, when a water solvent of a pyridine ring-containing chelate resin is replaced with an acetic acid solvent, the resin swells by about 1.7 times based on the packed volume thereof, and in treatment of the pyridine ring-containing chelate resin in an amount to be actually used in an industrial-scale terephthalic acid production process, attention should be paid to physical fracturing of the resin owing to rapid swelling thereof and to physical fracturing thereof owing to consolidation of the resins together.
Heat generation of the pyridine ring-containing chelate resin in the replacement of the water solvent with an acetic acid solvent is unexpected, and the details of the reason why the resin could generate heat are unknown. However, for example, it is known that, in replacing the water solvent of the pyridine ring-containing chelate resin filled in a column with an acetic acid solvent in an up-flow stream, the temperature of the chelate resin layer rises by about 30° C. depending on the condition, and such heat generation has some negative influences on the pyridine ring-containing chelate resin of which the heat resistance is problematic.
Regarding the heat resistance of a pyridine ring-containing chelate resin, a pyridine ring removal test is described in a literature (see PTL 2), in which the resin is added to a solution of 90-mass % acetic acid/10-mass % water kept boiling at a temperature of 110° C., and after 140 hours, the nitrogen concentration in the solution is measured to determine the pyridine ring removal rate from the resin, and it is known that the pyridine ring is removed by heat.
Regarding the generation of air bubbles, when a pyridine ring-containing chelate resin is filled in a column, there is an undesirable possibility of channeling generation and increase in pressure difference in the resin layer owing to the remaining air bubbles.
Regarding literatures relating to a pyridine ring-containing chelate resin, there are a literature relating to a production method for the resin (see PTL 3), a literature relating to a method for selectively removing a metal ion from a solution by the use of the resin (see PTL 4), and a literature relating to a method of recovering an oxidation catalyst by the use of the resin (see PTLs 1 and 5).
Regarding the description relating to swelling and contraction of a pyridine ring-containing chelate resin, for example, PTL 6 describes “it is not preferred that the degree of crosslinking is lower than 10%, because when the degree of crosslinking is lower than 10%, the resin structure may become greatly swollen or contracted by a reaction solvent such as acetic acid or the like, thereby causing break, deterioration or the like”. In addition, PTL 7 describes “the resin was formed into a slurry with water in the ion-exchange column. The resin swelled, . . . ”. Further, PTL 8 describes “it is considered that, when the value of the resin volume expansion rate is more than 20%, the change in the physical structure of the resin carrier would become great in such a degree that the effect of enhancing the heat-resistant stability and the abrasion resistance of the resin carrier could not be expressed”. However, the descriptions in PTLs 6, 7 and 8 relating to swelling and contraction of the resin are general matters, and the literatures do not refer to swelling and contraction of a pyridine ring-containing chelate resin in replacement of the water solvent for the resin with an organic solvent (methanol, acetic acid, etc.).
On the other hand, PTLs 3 and 5 describe replacement of the water solvent of a pyridine ring-containing chelate resin with an organic solvent (methanol, acetic acid, etc.), but have no description at all indicating, as a result of the replacement, swelling of the resin, heat generation of the resin or air bubbles generation.
PTL 4 has a description relating to swelling, saying that “the stage of contact is carried out by making the solution run through the resin layer in the up-flow direction to thereby swell the resin bed”, however, this relates to the contact method in removal of a metal ion from the solution, but does not relate to a pretreatment method of a pyridine ring-containing chelate resin.
Further, relating to pretreatment of a pyridine ring-containing chelate resin, PTL 1 has a description as follows: “For brominating a pyridine ring-containing chelate resin, for example, but not limited thereto, there is a method of bringing the resin into contact with an aqueous solution of the above-mentioned bromine compound such as sodium bromide, hydrogen bromide or the like or with a mixed solution of said aqueous solution with acetic acid, followed by washing it with glacial acetic acid or with water-containing acetic acid having a water concentration of at most 15% by mass to remove the excessive bromine”. However, neither does the literature disclose the phenomenon of swelling of the pyridine ring-containing chelate resin, heat generation of the resin or air bubbles generation in pretreatment of the resin.